1. Field of the Invention
The present invention relates in general to data transmission systems, and in particular to data transmission systems which may be utilized in wellbores to communicate remote control signals through fluid columns disposed therein.
2. Description of the Prior Art
In the oil and gas industry, it has been one longstanding objective to develop data transmission systems which do not require the utilization of electrical conductors to carry control signals between wellbore locations which are separated by great distances. Experience has revealed that data transmission systems which require the utilization of electrical conductors extending between communication nodes in a wellbore are advantageous when data must be communicated within the wellbore at extremely fast transmission rates, or when large blocks of data need to be transferred between communication nodes; however, the utilization of electrical conductors has several serious disadvantages including: (1) since most wellbores include regions which are exposed to corrosive fluids and high temperatures, a long service life cannot be expected from a data transmission system which utilizes electrical conductors; (2) since most wellbores extend for substantial distances, data transmission systems which utilize electrical conductors are not generally considered to be cost effective, particularly when such systems are utilized only infrequently, or in a limited manner; (3) since all wellbores define fairly tight operating clearances, utilization of a wireline conductor to transmit data may reduce or diminish the operating clearance through which other wellbore operations are performed; and (4) since wellbores typically utilize a plurality of threaded tubular members to make up tubular strings, utilization of an electrical conductor to transmit data within the wellbore complicates the make-up and break-up of the tubular string during conventional operations.
Accordingly, the oil and gas industry has moved away from the utilization of electrical conductor data transmission systems (frequently referred to as "hardwire" systems), and toward the utilization of pressure changes in a fluid column to transmit data within the wellbore. One example of the extensive use of fluid columns within a wellbore to transmit data is that of measurement-while-drilling data transmission systems, also referred to as "MWD" systems. Typically, these systems are utilized only in drilling operations. Generally, a plurality of sensors are provided in a tubular subassembly located within the bottom hole assembly, near the rock bit which is utilized to disintegrate the formation. The electrical sensors detect particular wellbore parameters, such as temperature, pressure, and vibration, and develop electrical signals corresponding thereto. The electrical signals are converted into a digital signal stream (generally multiplexed sensor data) and utilized to develop a plurality of pressure changes in a fluid column, typically the tubing fluid column, which are sensed at the earth's surface and converted into a format which allows the drilling engineers to make decisions which affect the drilling operations. Some attempts have been made to apply the concepts of MWD data transmission systems to completion operations, during which the drilled wellbore is placed in condition for continuous production of oil and gas from selected wellbore regions.
One of the more interesting of the prior art approaches is that described and depicted in U.S. Pat. No. 3,227,228 to Bannister. The Bannister reference is directed to a method and system for remotely actuating coring devices which are located in a drillstring. The coring devices may be individually and selectively actuated from a surface location, and function to automatically obtain core samples from the wellbore formation surrounding particular portions of the drill collar. The invention of the Bannister reference is succinctly summarized at Column 1, commencing at line 58, as follows:
"These and may other objects and advantages of my invention are accomplished in one embodiment by having one of the drill collars (hereinafter called a coring collar) in the drill string of a rotary rig contain a plurality of sample-taking devices and means for firing the devices in response to a remotely located wave energy source. The wave energy can be a controlled vibration of the drill string, a radio wave transmission, or a pressure variation transmitted down the drill mud. . . . When a formation sample is to be taken the operation of one or more coring devices is selectively controlled by wave energy transmission from a remote location." PA1 "The coring collar 20 mounts a number of coring devices 21 that are fired by firing selector 22 (FIG. 2) controlled by wave energy transmitted from a wave energy source at the surface to receiver 23 located in coring collar 20. The coring devices 21 can be fired selectively at any desired formation level." PA1 The coring devices 21 can be fired by several forms of controlled wave energy originating from wave energy source at the surface. The wave energy can be a vibration transmitted down the drillstring 8 from a vibration wave energy source 40 (FIG. 1), a pressure variation from a pressure wave energy source 41, or an electromagnetic transmission from a radio wave source 42. Each of these wave energy sources is conveniently associated with a rotary rig without interfering or significantly delaying the operation, as will be described hereinafter. PA1 The coring devices can be fired by a pressure vibration having a distinctive characteristic transmitted down the drill mud 6. A pulse or a wave having a preset frequency can select which coring device is to be fired. PA1 One embodiment of a pressure pulse firing system is illustrated in FIGS. 1 and 6. The pressure pulse source 41 fires an explosive charge 75 when switch 76 is closed in a fluid filled explosion chamber 77 connected through valve 73 to the stand pipe 7. Drill mud circulation is stopped, normally closed valve 78 is open and normally open valve 79 and 85 in the inlet and outlet pipe 17 and 15, respectively, are closed. The explosion creates a steep front, high amplitude pressure variation that travels down the drill mud 6 inside drillstring 8 to the coring collar 20. PA1 Disposed within coring collar 20 is a pressure responsive receiving means 80 (FIG. 6) that actuates a firing selector 81 to selectively fire the coring devices 21. The pressure variation flexes a diaphragm 94 disposed in the wall of passage 30, transmitting a force through an incompressible fluid 82 to a piston 83. An outward force (to the left as viewed in FIG. 6) is applied to piston 83 by a spring 84 keeping contact 86 at the end of piston rod 87 from stationary contact 88. When the high amplitude pressure variation reaches the coring collar 20, the contacts 86 and 88 close an energizing circuit including battery 89 to a solenoid 96 that operates a pawl 90 and rotates a ratchet wheel 91 to a new position. At each position of ratchet wheel 91 an attached contact arm 92 connects with a fixed contact that closes an energizing circuit including battery 93 to fire the electric detonator in one of the coring devices 21. Each pressure pulse fires another one of coring devices 21 as the ratchet wheel 91 is progressively moved to new positions. PA1 Another form of pressure responsive receiving means that uses electronic techniques to duplicate the above describe electromechanical system is illustrated in FIG. 7. PA1 Another pressure wave firing system suitable for use in the present invention utilizes a pressure source that generates an alternating pressure variation in a single frequency in the drill mud 6. The pressure wave responsive receiver includes a pressure variation transducer that produces an A.C. signal from the transmitted pressure wave, using a filter channel to fire one of the coring devices. As in the mechanical vibration arrangement, other frequencies and filter channels can be incorporated to selectively fire additional coring devices 21. PA1 It is apparent that the vibration wave system previously described can be modified to operate on a series of pulses as in the pressure responsive system embodiment just described with reference to FIG. 7. In such an arrangement only one frequency need be used and the A.C. generator 45 would be connected to vibrator 47 only for a moment to produce each vibratory pulse. The vibration response receiver would include a band-pass filter preferably following amplifier 101 that responds only to the selected frequency.
The Bannister reference teaches the alternative utilization of three techniques for remotely controlling the firing of the coring devices in a coring collar. Those techniques include (1) applying vibration energy to the drillstring, (2) utilizing a pressure pulse generator to alter the pressure in the fluid column, or (3) utilizing a radio transmitter. All three of the alternative actuation techniques are depicted graphically in FIG. 1 of the Bannister reference. The vibratory energy supply 40 is depicted as being directly mechanically linked to the drillstring. The radio transmitter 42 is depicted as utilizing an antenna to transmit radio frequency actuation signals. The pressure pulse generator 41 is shown as communicating with the flowlines of the drilling rig, to allow the direct application of the pressure pulses to the fluid column in the wellbore. The Bannister reference uses the term "wave energy" to encompass all three types of alternative actuation systems. The broad objective of the Bannister invention is stated at Column 3, commencing at line 30, as follows:
This is further elaborated on commencing at Column 3, line 74, as follows:
In the figures of the Bannister reference, the vibration transmitter is depicted in FIGS. 3, 4, and 5. The pressure pulse generators (two alternative embodiments) are depicted in FIGS. 6 and 7. A radio frequency actuation apparatus is depicted in FIG. 8. The remaining figures (FIGS. 9 through 14) depict the mechanical components of the coring tool itself.
The pressure pulse transmission equipment is described in the specification between Column 4, line 66 and Column 5, line 68. Bannister plainly teaches the utilization of a "distinct characteristic" in the pressure signals, as stated at Column 4, commencing at line 66, which states as follows:
It is also clear from the Bannister reference that a high velocity pressure change is contemplated. In all probability, the pressure change can be characterized as an acoustic pulse. The specification clearly states this commencing at Column 4, line 70, which states as follows:
Thus, it appears that the pressure pulse is probably traveling close to the velocity of sound for the particular transmission medium. The two different types of pressure pulse generators which are depicted in FIGS. 6 and 7 are described separately in Column 5 of the Bannister reference. The embodiment of in FIG. 6 is described as follows:
The alternative embodiment of the pressure pulse generator of FIG. 7 is described as follows, commencing at Column 5, line 25:
The present data miniaturization of electronic components facilitates the compact arrangement of this apparatus, wherein the pressure variation is sensed by a pressure responsive transducer 100, preferably a piezoelectric device, and a voltage proportional to the pressure, after being amplified by amplifier 101, is coupled to a threshold limiter 102. The threshold limiter serves to prevent normal pressure variations in the drill mud 6 from firing the coring device 21 by producing an output signal only if the pressure-proportional input voltage exceeds a preset minimum. The Schmidt trigger circuit is one suitable type of threshold limiter, producing for each input pulse about (sic) a preset level an output pulse that is coupled to a univibrator 107, (a mono-stable multivibrator) to produce a pulse that is amplified in amplifier 103. Each pulse activates a stepping switch 104 having an input to successively connect an input 105 to each of outputs 106, closing an energizing circuit including battery 108 for the electric detonator of one of the coring devices 21.
Yet another alternative system, which is not depicted in the drawings, is discussed for use in pressure pulse actuation, commencing at Column 5, line 58 which reads as follows:
The particular reference to the "mechanical vibration arrangement" is an identification of the foregoing text which relates to the mechanical vibration actuated firing, which commences at Column 5, lines 48, which states as follows: